All closed containers or vessels have access openings to the internal parts of the vessels. The access openings are variously known as manholes, hatches, and so on. The manhole is formed by a manhole frame. The configuration of the manhole frame varies in accordance with the geometry of the tank.
The subject of the present invention is a manhole frame for a spherical tank of the type used for underground storage of fluid materials, typically petroleum products. Spherical tanks in the current art are generally made of a filament, generally fiberglass, which is wound around an inflatable and deflatable ball, or mandrel. Manhole frames are prepared in advance and then fitted over the inflated mandrel. The spherical tank is then formed by a glass strand roving process known in the art as filament winding that incorporates the manhole frame, which becomes an integral part of the formed spherical tank. After completion of the spherical tank, the mandrel is collapsed and withdrawn from the tank through the manhole.
The old art of producing manhole frames for spherical tanks, which were formerly made of steel as were the tanks, is generally followed in the current art even though the spherical tanks and their manhole frames are made of nonmetal moldable materials rather than of metal. Both the old art and the present art use injection molds, or dies, to produce the manhole frames.
Two basic types of manhole frames are known in the art. These types are shown in FIGS. 1 and 2, which are discussed in detail later. Both of these manhole frames follow the types used when spherical tanks were made of metal rather than fiberglass or plastic. The first type of manhole frame, which is illustrated in FIG. 1, includes a neck with a collar and bolts integral with the neck for later attachment with a closure plate. The bolts tend to loosen because of an inherent inability of the bolts to be firmly held by the resin that surrounds them with resultant leaking and maintenance expenses later. Another problem arises during molding when the resin tends to run downwardly onto the bolt threads. The second type of manhole frame, which is illustrated in FIG. 2, includes a neck connected to a manhole flange having bolt holes for bolts to be added later. This type of frame must be reinforced at the joint between the neck and the spherical tank by added spraying around the neck at added cost. Problems with leakage at the reinforcing arise with associated maintenance expenses.